The study site is equipped with a variety of sensors, including piezometers, inclinometers, and time domain reflectometry probes (TDR), to monitor subsurface movements and soil properties. So far, the study site is equipped with the following installations:
- 3 Piezometer
- 9 TDR probes (3 x [0.5 m, 1 m, 2 m])
- 5 Inclinometers (4.5 m - 6 m depth)
All data recorded by the permanent installations are collected every 5 min and are sent directly to the server in Vienna, where these are processed. Moreover, the collected data can be viewed in real-time at the webpage.
In total, 3 piezometers were installed in the summer of 2022 to measure underground water pressure. Put simply, a piezometer is a tube that is inserted into the soil to depths below the water table. The tube extends to the soil surface and is therefore open to the atmosphere. As the bottom of the piezometer is perforated, soil water under positive hydrostatic pressure can enter the tube (Or et al. 2005). Water that entered the tube rises to a height qual to that of the unconfined water table and can be measured relative to the soil surface (ibid.).
Time domain reflectometry (TDR)
To obtain further information on soil moisture, a total of 9 TDR probes were installed at three locations, in three different depths (0.5, 1 and 2m) respectively. TDR is an electromagnetic measurement technique to determine the dielectric permittivity and electrical conductivity of a variety of porous materials and is employed in a wide range of monitoring applications (Comegna et al. 2022;). Most frequently, TDR is applied to measure the volumetric water content in soils (Comegna et al. 2022, Cataldo et al . 2021). Volumetric soil water content can be measured indirectly based on the charge-storing capacity of the soil. Each component of soil has a unique ability to store electrical charge, also called the dielectric constant. Water has by far the highest dielectric constant out of all components of a soil. Therefore, measurements of the charge-storing capacity can be related to the volumetric water content (Comegna et al. 2022).
Subsurface movements are monitored using manual inclinometers. In total, five inclinometers ranging from depths of 4.5 to 6m were installed at the study site. Inclinometer casings are grooved plastic pipes inserted into a borehole and can be used to monitor lateral movements of slopes (Indraratna et al. 2015). Inclinometer data are collected using a measuring probe and a manual data logger during field trips performed roughly every four weeks. The zero measurements took place on August 18th, 2022.
Dynamic probing and percussion drilling
In addition to these permanent installations, the subsurface is also explored through dynamic probing and percussion drilling. For Brandstatt, a total of 26 deep penetration heavy (short, DPH) soundings and 8 percussion drillings were carried out in the summer of 2022.
During a dynamic probing test, a metal rod is gradually driven into the ground by means of weight. The number of blows required per certain penetration depth, e.g., per 10cm depth, is recorded. This technique provides information on resistance parameters (bulk density, consistency, etc.), but also on the maximum penetration depth (bedrock boundary). To perform DPH soundings, a drop weight of 50kg is used.
To obtain information on the stratigraphy of the subsurface structure, drill cores were extracted. For percussion drilling a hollow steel tube is driven into the ground by means of a weight. The removal of drill cores, i.e., soil samples, enables the analysis of various parameters such as grain size distribution. In addition, various sensors can be installed in the resulting boreholes.
Fig. 1: A & C: Percussion drilling using a drilling rig. B: Inclinometer measurement. © R. Kanta. 2022.
Cataldo, A., E. De Benedetto, A. Masciullo, G. Cannazza. 2021. A new measurement algorithm for TDR-based localization of large dielectric permittivity variations in long-distance cable systems. Measurement, Vol. 174, 109066.Comegna, A., G. Severino, A. Coppola2022. A review of new TDR applications for measuring non-aqueous phase liquids (NAPLs) in soils. Environmental Advances. Vol. 9, 100296.Indraratna, B., I. Sathananthan, C. Bamunawita, A.S. Balasubramaniam. 2015. Chapter 3 - Theoretical and Numerical Perspectives and Field Observations for the Design and Performance Evaluation of Embankments Constructed on Soft Marine Clay. Ground Improvement Case Histories, Butterworth-Heinemann, 83-122.
Or, D., M. Tuller, J.M. Wraith. 2005. Water Potential. Encyclopedia of Soils in the Environment, 270-277.